The effects of PMSG treatment, estrogen primings or hypothalamic lesions on an acceleration of pubertal processes were investigated in the female guinea pig and rhesus monkey in order to determine the mechanisms of the onset of puberty. Electrolytic lesion of the posterior hypothalamus accelerated the onset of menarche in the monkey. This finding indicates that the hypothalamus inhibits an initiation of puberty until maturation of the CNS-pituitary-ovarian axis. Prior to the onset of menarche, the establishment of the positive feedback loop of ovarian steroids seems to be withheld by the hypothalamus; estrogen-induced LH surges were not observed in premanarchial monkeys. However, at the time of puberty, the hypothalamus facilitates the pubertal process (menarche through the first few ovulations) by supplying LHRH with diurnal rhythmicity. Diurnal rhythmicity of serum LH was apparent only in the period starting just before the onset of menarche and diminishing shortly after the first few ovulations in the post-pubertal period. Thus, our results indicate that the hypothalamus plays a major role in the onset of puberty in the rhesus monkey. Furthermore, somewhat similar results obtained in the guinea pig on pubertal mechanisms suggest that the guinea pig may be a useful model for primate research.